1. Field of the Invention
The present invention relates to a sliding member. The sliding member includes a member such as a spline portion of a propeller shaft (a drive shaft), a cage or a raceway surface of an inner or outer ring of a bearing, or a clutch plate of a friction clutch, for example.
2. Description of Related Art
A vehicle is loaded with a propeller shaft (a drive shaft) for transmitting driving power from the engine to a differential gear. The propeller shaft is formed by coupling two shafts with each other to be expandable/contractible in the axial direction. More specifically, an external spline portion consisting of a plurality of keys along the axial direction and an internal spline portion consisting of a plurality of keyways are formed on the coupled portions of the shafts respectively. The spline portions slidably engage with each other, so that rotational driving force of one of the shafts is transmitted to the other shaft and the overall propeller shaft can be expanded/contracted. The spline portions are filled with a lubricant such as grease. If the lubricant runs out, a stick slip (hang-up) may be caused in sliding of the spline portions, to result in noise.
A structure of covering the spline portion of each shaft with a DLC (Diamond Like Carbon) film having wear resistance (high hardness) and low frictional properties so that occurrence of such noise can be suppressed even if the lubricant runs up is known (Japanese Unexamined Patent Publication No. 2006-22882, for example).
However, the DLC film has extremely high internal stress. If the DLC film is directly formed on the surface of a base material, therefore, the DLC film may be easily separated or cracked due to low adhesiveness of the DLC film with respect to the base material (particularly a metal substrate).
Therefore, a technique of interposing an intermediate layer such as a Ti (Titanium) layer between the base material and the DLC film thereby improving the adhesiveness of the DLC film with respect to the base material and increasing the life thereof is known in general (Japanese Unexamined Patent Publication No. 2002-235748, for example).
However, the intermediate layer is constituted of a single film, and hence the adhesiveness of the DLC film with respect to the base material is insufficient. Therefore, awaited is a means for further improving the adhesiveness of the DLC film with respect to the base material by providing the intermediate layer in a multilayer structure.